Another good way to demo this is using a cylindrical magnet and drop it
through a copper tube like used for plumbing. Re your sorting method I
believe this is similar to how many vending machines validate that a
coin is what it should be rather than a counterfeit.

Dave Hinz wrote:

On Mon, 24 Apr 2006 01:08:33 +0000, Christopher Tidy wrote:

David Billington wrote:

It would be interesting to know what granite was used for the base of
your height gauge. The black granite I have about shows no magnetic
attraction at all when NiFeBn magnet is placed on it. As granite is
predominantly SiO2 it would be interesting to know why the magnetic
attraction.

Given the strength of these magnets I could believe it was just being
attracted to a steel fitting in the centre of the height gauge base. Is
this possible, or was it definitely being attracted to the granite?


Another thing to consider is eddy currents. Take a hard drive magnet
assembly, one with the self-contained yoke and magnets on both sides.
Old seagate drives, for instance. If you drop a dime through it, it
slows down noticably while falling through the gap. A quarter, more so.
A half dollar, takes nearly 1 second to drop through the gap (really!)

If you're wondering, this is related to my "how to I separate zinc
pennies from copper pennies" question of last week. As soon as I can
get a good ramp/guide, I should have some wacky pictures of my low-tech
sorting method.

Dave Hinz